The present invention is directed to a method of recovering catalytic metal. More specifically, the present invention is directed to a method of recovering catalytic metals from fluid compositions containing catalytic metal colloids using a porous metal filter.
Electroless metal deposition refers to the chemical deposition of a metal on a conductive, non-conductive, or semi-conductive substrate in the absence of an external electric source. Electroless deposition is used for many purposes, for example, in the manufacture of printed circuit boards where, in one method, an electroless metal, often copper, is deposited on a dielectric substrate either as a uniform surface coating or in a predetermined pattern. The initial electroless copper deposit is thin and may be further built up by electroplating or may be deposited directly to fill thickness.
The substrate over which an electroless metal deposit is formed is often a plastic panel which may have a metal foil such as copper laminated to one or both of its surfaces, for example, with adhesive, to form a metal clad substrate. Where both surfaces of the substrate are to be used, connections are provided therebetween by means of holes through the panel at appropriate locations. The walls of the holes are made conductive with electroless coating.
The electroless deposition of a metal on either a metallic or non-metallic substrate requires pretreatment or sensitization of the substrate to render it catalytic to reception of a metal deposit. Catalytic metal colloids are often used as the sensitizer or seeder to prepare the substrate for reception of the metal.
Catalytic metal colloids are dispersions formed by the admixture of a catalytic metal ion and a non-catalytic metal ion in an amount in excess of the catalytic metal ion. Such dispersions are often formed in acidic solutions but also may be formed in alkaline solutions. Suitable catalytic metal ions are well known in the art. Examples of highly desirable catalytic metal ions are the noble metal ions of gold, platinum and palladium. An example of a suitable non-catalytic metal ion used to form the metal colloid is stannous metal. Colloidal baths or solutions may contain tin in amounts of from about 10 to about 50 or more times than the amount of catalytic metal. Typically, a catalytic metal such as palladium may range in concentrations of from about 140 ppm to about 150 ppm in the colloid bath. Such catalysts are commercially available. U.S. Pat. No. 3,011,920 to Shipley, Jr. discloses methods of making such catalysts, the disclosure of which is hereby incorporated in its entirety herein by reference. Also, U.S. Pat. Nos. 4,020,009 and 4,085,066 both to Gulla and assigned to Shipley Company, Inc. disclose catalytic metal colloids and methods of making the same, the disclosures of which are hereby incorporated in their entireties herein by reference.
Prior to electroless metal deposition on a substrate, such as a printed circuit board, the part of the substrate to be plated is immersed in a colloidal bath or solution. The substrate is then rinsed with water and then placed in an electroless bath for plating. About 70% or more of the catalyst consumed by the substrate during immersion is washed off of the substrate by the rinse. Thus, about 30% or less of the catalyst remains on the substrate. The catalytic metal colloids represent a major cost in electroless metal deposition. Thus, recovering the catalytic metal colloids for reuse is highly desirable. However, recovery of the catalytic metal from the rinse is difficult because the catalytic metal is in small concentrations and the non-catalytic metal, such as tin, is present in large concentrations. Thus, the rinse is often discarded with the loss of the valuable catalytic metal.
In addition to the loss of catalytic metal from rinses, catalytic metals also may be lost from the catalytic metal colloidal solutions or baths. For example, when employing copper clad substrates, such as printed circuit boards, which are drilled to provide through-holes, the through-holes are metal plated to provide a continuous current path when individual boards are joined together. Because the exposed surfaces in the holes are non-metallic, electroless plating techniques including the step of catalyzing by means of a catalytic metal colloid, such as tin/palladium colloid catalyst, is employed. Copper clad boards are immersed in the catalytic bath to deposit the catalyst thereon. Copper from the copper clad boards contaminates the catalytic metal colloidal bath with continued use of the bath. When the contamination reaches an extent such that the bath becomes ineffective or the electroless plating becomes less adherent than desirable, the bath is xe2x80x9cspentxe2x80x9d and is then discarded as waste.
Because many of the metals employed in the catalytic metal colloids are costly, especially gold, platinum and palladium, industries, such as the printed circuit board industry, would prefer to recover the metals rather than dispose of them. Recovery of the metals would reduce manufacturing costs to manufacturers of printed circuit boards and reduce costs to the manufacturers"" customers. Also, the catalytic metals present a hazard to the environment, and disposal of the metals is strictly regulated by the Federal and State governments. Often large volumes of liquid waste are transported far distances to designated sites for proper disposal. Thus, proper disposal procedures for the metals are costly to the industry and much of the cost is passed onto the customer. Although recovery of catalytic metals from catalytic metal colloids is highly desirable, an economically efficient method for the recovery of the catalytic metal from colloids has not been developed. Accordingly, there is a need for an economically and environmentally safe method for recovering catalytic metals from colloidal metal catalysts.
A few attempts have been made to recover catalytic metals from waste solutions. U.S. Pat. No. 4,435,258 to Milka, Jr. et al. and assigned to Western Electric Co., Inc. discloses a method of recovering palladium from spent electroless catalytic baths employing an electrolytic cell. The method of recovery disclosed in the ""258 patent involves (a) dissolving tin/palladium colloid in a spent catalytic bath with an oxidizing agent such as hydrogen peroxide to form a true solution; (b) heating the bath to a temperature and for a time sufficient to essentially remove excess hydrogen peroxide; (c) placing the solution in an electrolytic cell having (1) a nickel anode, and (2) a cathode composed of a metal or metallic surface, such as copper or nickel, for the palladium to be deposited; and (d) electrodeposition of palladium from the solution onto the cathode at a voltage that allegedly tends to minimize and substantially reduce tin deposits. There are many disadvantages with such a method. Electrolytic cells can be costly. The consumer of the palladium colloid either has to invest in purchasing such electrolytic cells, or pay the cost of transporting the spent catalytic bath to a site where the electrolytic cell is located. Because of the weight of fluids, the cost of transporting the bath to the recovery site is expensive. If the consumer purchases the electrolytic cell, then the consumer must expend funds in both operating and maintaining the cell. Such an electrolytic cell as described in the ""258 patent is specially designed and replacement of worn parts may not be inexpensive or readily obtainable. For example, the electrolytic cell of the ""258 patent has a specially designed cascading structure to allegedly prevent deposited palladium from breaking away from the cathode. Also, a high purity nickel anode and cathode are recommended to obtain acceptable recovery amounts of palladium. Such adds to the cost of the apparatus. Amounts of palladium recovered also depend on the amounts of specific components in the colloidal bath as well as any contaminants. The more dilute the palladium and the more contaminants in the bath the more difficult the recovery of the palladium. Such contaminants as copper salts or other metal contaminants may compete for deposition at the electrodes with the palladium. Several palladium colloidal catalysts are obtainable from commercial sources and the specific components and purity vary. Thus, the efficiency of such electrolytic cells may vary. Another problem associated with such electrolytic cells is duration of operation. High recovery of palladium by an electrolytic cell often requires many hours of operation. Such long hours of operation increase the cost of recovering catalytic metal and add wear to the electrolytic cell.
Research Disclosure 31448 (anonymous, June 1990) entitled xe2x80x9cReclamation of Palladium from Colloidal Seeder Solutionsxe2x80x9d discloses a method of recovering palladium from colloidal tin/palladium solutions used to promote electroless metal depositions. The palladium is recovered by flocculating the colloid by rapid mixing with air or oxygen. The oxygen allegedly does not oxidize the palladium. A palladium rich precipitate is allegedly obtained. The precipitate is dried and further processed. The document is silent on the further processing of the precipitate to recover the palladium as well as the efficiency of the disclosed method. The document only mentions that the method is intended to eliminate costly trucking of the hazardous waste from the colloidal solutions.
U.S. Pat. No. 5,302,183 to De Boer et al. and assigned to Shell Oil Company discloses a method of recovering precious metals such as platinum and palladium from non-aqueous effluents in colloidal and/or dissolved states. Such effluents are from non-aqueous effluents leaving flow-through reactors or bleed streams from a stripping reactor, not from aqueous solutions of colloidal catalysts or aqueous rinses as employed in the circuit board industry. The non-aqueous colloidal metal and/or dissolved metal effluents may be initially distilled to remove unwanted reaction product in the effluent. The non-aqueous effluent also may be dried to remove any water or the effluent may be filtered. The patent is silent on the specific method or efficiency of the filtering method. If the aforementioned steps are eliminated, the non-aqueous effluent may be immediately reduced with a reducing agent. The reducing agent is added to the non-aqueous effluent to complete reduction of any cationic precious metals present in the non-aqueous effluent. Suitable reduction agents are carbon monoxide and lower olefins such as ethylene. The reducing agents are contacted with the non-aqueous effluent in a gaseous state.
After reduction, the reduced precious metal is deposited on a support such as activated carbon or porous granular plastic or resin. The reduced precious metal deposited on the support may be recovered by filtration, decanting, centrifugation or the support may be burned and the precious metal transported to the appropriate facilities for further processing.
Although the ""183 patent alleges a high recovery of precious metal from the disclosed process, the process suffers from a number of disadvantages. First, the reducing step employs expensive technological equipment such as gas chambers to apply the reducing agent in gaseous form to the non-aqueous effluent. Such a step involves transporting the non-aqueous effluent to a facility having such equipment, or the purchase and maintenance of such equipment by the workers where the non-aqueous effluent is recovered. Additionally, trained workers are employed in the operation of the equipment used in the reducing step adding to the cost of the process. Thus, the reduction step is costly. Further, carbon monoxide is a preferred reducing gas. Carbon monoxide gas is very toxic and presents a hazard to workers performing the reduction process. The other reducing agent, i.e., the lower olefins, also may present a hazard to workers. For example, ethylene presents a serious flammability problem. Additionally, the ""183 patent is limited to recovering precious metals only from non-aqueous effluents.
Adsorbents such as resins are known in the art to be used for recovering precious metals from aqueous solutions. A paper entitled xe2x80x9cExtraction and Recovery of Precious metals from Plating solutions Using Molecular Recognition Technologyxe2x80x9d by S. R. Izatt et al. discloses the use of SuperLig(copyright) 127 resin for selectively recovering potassium gold cyanide from drag out rinse solutions, and SuperLig(copyright) 2 resin for recovering palladium metal from dipping baths. SuperLig(copyright) resins are proprietary crown ether resins obtainable from IBC Advanced Technologies Inc., of American Fork, Utah. A disadvantage in the method for recovering potassium gold cyanide by the method using SuperLig(copyright) 127 resin is that a concentrator with a vacuum and a heat exchanger is employed to concentrate the potassium gold cyanide from drag out rinse solutions to a concentration of 16 g/l. Such apparatus adds to the cost of the process. A disadvantage of both the potassium gold cyanide and the palladium recovery processes is the limiting of the recovery processes to the use of a specific proprietary resin. A worker practicing the method is restricted to using a specific proprietary resin without an alternative material for recovering the metals. Thus, the method is inflexible for the worker. Also, such resins are costly to manufacture and often require skilled workers to operate the resins and maintain them. Another problem with employing resins, in general, is that the resins may become fouled with salts, non-catalytic metals and undesirable precipitated solids during the recovery process. Thus, the resins have to be regenerated or replaced with new resins to continue the recovery process. The added step of regenerating the resins delays the recovery process. Also, some of the catalytic metal mixed with the materials that foul the resin may be lost during regeneration. Replacing the fouled resin with new resin adds to the cost of the recovery process. Accordingly, there is a need for a more economic and flexible method for recovering catalytic metals.
U.S. provisional patent application Ser. No. 60/262,592 filed Jan. 18, 2001 discloses an efficient method of recovering catalytic metals from solutions containing catalytic metal colloids. The method involves recovering catalytic metal colloids from solutions by capturing the colloids on a filter as a precipitate followed by washing the precipitate with an oxidizing agent until the catalytic metal is removed from the filter. The catalytic metal is recovered in a separate container and then collected on an adsorbent. The adsorbent is burned and the catalytic metal is retrieved. The filters used to collect the catalytic metal colloid are disposed of. Although the method provides an efficient means of recovering catalytic metal, there is still a need for an improved method.
The present invention is directed to a method of recovering catalytic metals from a fluid containing catalytic metal colloids by concentrating the catalytic metal colloids as a precipitate on a porous metal filter followed by removing the precipitate from the porous metal filter by backwashing the filter with a fluid, solubilizing the precipitate, and then retrieving the catalytic metals.
Advantageously, the method of the present invention provides an economically efficient means of recovering catalytic metals for reuse. Filtering the catalytic metal colloid species from a fluid with a porous metal filter concentrates the catalytic metal colloid as a precipitate from many other components of the fluid that may interfere with catalytic metal recovery or increase both the time and expense for recovery. Such other components may be excess non-catalytic metal, salts, contaminants from the printed wiring boards, and the like.
Catalytic metal colloids are employed in metal deposition processes and compositions. Such metal deposition compositions include electrolytic and electroless solutions, i.e., solutions capable of the chemical deposition of an adherent metal coating on a conductive, non-conductive, or semi-conductive substrate in the absence of an external electric source. The part of the substrate to be plated with a metal is contacted with a catalytic metal colloid solution or bath to coat the substrate with the colloid. The catalytic metal colloid acts as a seeder for metal deposition on the substrate. The substrate may then be placed in a metal plating solution for metal deposition. The substrate may be rinsed a number of times during the process. Some of the catalytic colloid is carried away in the rinse. Such rinse solutions are also known as drag out baths. Because the catalytic metal colloids represent a major cost of operating metal deposition processes, recovering the catalytic metal is highly desirable. However, because the amount of catalytic metal in catalytic metal colloids is proportionately very small in relation to non-catalytic metal, workers in the industry have been discouraged from recovering the catalytic metal, or have not found a satisfactory method of recovering catalytic metals efficiently. The amount of non-catalytic metal in a colloid may be from about 10 to as much as about 50 times the amount of catalytic metal. Further, catalytic metal colloids are employed in solutions in very dilute amounts. Thus, recovering catalytic metals from such dilute solutions makes the task even more difficult, and in many instances economically inefficient. Continued loss of catalytic metals is costly to the industry.
Filtering solutions with a porous metal filter captures and concentrates catalytic metal colloids as a precipitate on the filter. The porous metal filter allows much of the excess soluble non-catalytic metal, plating metals, metal salts, complexing ions, reducing agents, alkali metal salts, pH adjusters, brighteners, stabilizers, and other components in the rinse solutions to pass through the filter. Thus, employing a porous metal filter in a filtering step provides a rapid and efficient means for concentrating and recovering catalytic metal colloids. Advantageously, porous metal filters concentrate catalytic metal colloids from dilute fluids or solutions as a precipitate such that most of the catalytic metal may be recovered. The precipitate may be removed from the porous metal filter by backwashing the filter with a fluid and the precipitate collected in a suitable container. The precipitate may then be solubilized and the catalytic metal collected on a suitable adsorbent. The adsorbent may then be burned and the catalytic metal collected.
The method of the present invention is highly desirable for any industry where catalytic metal colloids are employed. The printed circuit board industry, where catalytic metal colloids are employed in metal deposition processes, especially benefits from the recovery method of the present invention. Expensive catalytic metals in dilute concentrations may be readily recovered without added complex time consuming steps. Thus, the method of recovering catalytic metals using a porous metal filter is economically efficient.
Additionally, the method of the present invention is environmentally friendly. The method of the present invention provides a high recovery of catalytic metals that are potentially hazardous to the environment. Also, the filtered catalytic metal colloids may be transported to another site for further processing without the expense or danger of spillage as with large volumes of hazardous fluid.
A primary objective of the present invention is to provide a method for recovering catalytic metals from fluids containing catalytic metal colloids using a porous metal filter.
Another objective of the present invention is to provide a method of recovering catalytic metals from a catalytic metal colloid solution by an economically efficient means.
A further objective of the present invention is to provide a method for recovering catalytic metals from a catalytic metal colloid solution that is environmentally friendly.
Additional objectives and advantages of the present invention may be ascertained by those of skill in the art by reading the detailed description of the invention and the appended claims.